Aquatic Weed Control

Emergent Weeds

Spray when the weeds are actively growing with a full emergence of green leaf from flowering and up to die-back. For the control of Watercress, the best results are from June applications. Bulrush from late July applications and for all the remaining species from mid-August to mid-September.

Leaf symptoms on the weeds begin with a grey/greening and then yellowing of the foliage usually appearing within 14-21 days of spraying in the early autumn. Complete foliage desiccation usually occurs 30-40 days after spraying. At this stage the weeds can be cut and removed. During cold conditions leaf symptoms may not appear before natural die-back but no growth will occur in the season following spraying.

Floating Weeds

Spray when there is a maximum emergence of floating leaves. For the control of Water Lilies, the best results are obtained from applications made from mid-July to mid-August. From 7-21 days after spraying the foliage turns yellow, gradually disintegrates and sinks below the surface of the water. A few leaves may remain green until the end of the season but do not regrow in the following year.

Planning a Weed Control Programme

Weeds are sometimes defined as plants growing where they are not wanted. Water plants are usually wanted, and become weeds only when their growth is excessive.

Before taking action, define the objectives of weed control, consider the consequences, and choose the most appropriate method.

Inappropriate weed control, whether chemical or mechanical, can harm the environment, leading to: poor species diversity; changes in the pattern of silt deposition; de-oxygenation of the water; and poisonous weeds becoming more palatable to grazing animals. It can also let in invasive weeds, which are more difficult and expensive to control.

Guidance on managing aquatic vegetation with particular reference to wildlife is available from English Nature, Scottish Natural Heritage and the Countryside Council for Wales.

Rates of Use

Roundup may be used for the control of aquatic weeds in the presence of fish if used in strict accordance with the recommendations in this section.

Target weedsRate of Roundup ProBioRate of Roundup ProVantageVolume of water for hydraulic sprayers
Emergent weeds 5 l/ha 3.75 l/ha
  • 200-400 l/ha
  • optimum 250
Floating weeds 6 l/ha 4.5 l/ha 100-200
Weed wipers

1 part Roundup Probio to 2 parts water

1 part Roundup ProVantage to 3 parts water


Emergent Weeds include:
Bulrush, Common Reed, Creeping Bent, Reed Canary Grass, Reed Sweet Grass, Sedges, Soft Rush, Watercress, Whorlgrass

Floating Weeds include:
White Water Lily, Yellow Water Lily


Any knapsack sprayer, tractor-mounted or boat mounted sprayer may be used provided it is capable of applying the appropriate spray volume accurately and at the correct pressure, to achieve a 'medium' or 'coarse' quality spray, (BCPC definition).

Avoid high water volumes, which may lead to run-off and loss of chemical.

Applications made in flowing water should be sprayed against the direction of flow.

Roundup must be applied as a directed spray to green, actively growing weeds and drift must be avoided.

Applications must be made before the leaves of the weeds have started to die back.

Hand-held weedwipers may be used to apply Roundup directly to weeds.

When operating in or near water, it can be difficult to get the spray nozzle over weeds growing at a distance from the bank. Consider using a telescopic hand-held lance, which extends to give a spray range of up to 5 metres. These lances are also useful for spraying tall bankside weeds such as Giant Hogweed and Japanese Knotweed.

All spray equipment should be checked and cleaned thoroughly before and after use, and should be calibrated regularly.

With a boat-mounted sprayer, use the slowest forward speed to cause minimum disturbance to the leaves of the weeds. When disturbed by the wash, water lilies may require retreatment.